The present invention relates to an exhaust gas turbocharger for an internal-combustion, and more particularly, to a turbocharger having a turbine wheel which is arranged in the housing of the exhaust gas turbocharger and to which exhaust gas can be fed by way of a flow duct constructed in the housing, and having a variable baffle for adjusting a flow cross-section of the flow duct.
DE 195 43 190 A1 discloses an internal-combustion engine which has an exhaust gas turbocharger with a variable turbine geometry. On a housing of the turbine of the turbocharger, an adjustable baffle is arranged which has guide blades between which flow ducts for the exhaust gas are constructed. A control element provides adjustment of the guide blades between an opening position with the lowest possible reduction of the flow cross-section of a flow duct feeding exhaust gas and a ram position with the highest possible reduction of the flow cross-section. In order to obtain a high braking effect during the braking operation of the internal-combustion engine, the baffle is changed to its ram position, whereupon an excess pressure builds up in the section between the cylinders and the exhaust gas turbocharger. The brake pressure results in a rise of air flow in the cylinder and counteracts the compression of the gas in the cylinder.
Simultaneously, exhaust gas flows at a high velocity through the flow ducts between the guide blades and acts upon the turbine wheel whose power is transmitted to the compressor which builds up an excess pressure in the intake system and feeds it to the cylinder.
The cylinder is therefore acted upon with an increased charging pressure on the input side. A quasi-static excess pressure exists on the output side between the cylinder outlet and the exhaust gas turbocharger. This excess pressure counteracts the blowing-off of the air compressed in the cylinder into the exhaust gas piping. In the braking operation, the piston must therefore carry out compression work during the compression stroke against the high excess pressure in the exhaust gas piping, whereby a strong braking effect is achieved.
DE 43 30 487 C1 also shows a turbine with a variable turbine geometry. The turbine has a radial and a semiaxial ring nozzle. A stationary baffle is arranged in the semiaxial ring nozzle, and a variable baffle with rotatable guide blades is arranged in the radial ring nozzle. In addition, a ring, which is contoured in a fluidically advantageous manner, is arranged on the stationary baffle, is situated between the semiaxial and the radial ring nozzle and forms a stationary non-adjustable component of the turbine.